A new paper finds that a carbon tax meant to shift agricultural policies could raise food prices and threaten food security.

However, improvements in storing carbon in the world’s soils could lessen the potential for worsening food security.

The researchers suggest a globally coordinated effort on climate-friendly agriculture and land use would likely result in the best outcome for all.

A new study in Environmental Research Letters shows that applying a theoretic carbon tax — one aimed at stimulating changes to farming and land-use practices that minimize emissions —could have a major impact on food security, resulting in as many as 300 million more people suffering food deprivation. But add soil carbon-friendly farming into the mix, and you could limit the impact on food security and reduce calorie loss by 65 percent while at the same time sequestering more carbon in the ground.

“Soil carbon sequestration can help to address climate change, and because it also helps to increase productivity, can also help to address food security,” said study co-author Peter Smith, an expert in soils and climate change from Aberdeen University in the U.K.

Agriculture is responsible for 10 to 12 percent of global greenhouse gas emissions. Given the growing global population, experts expect agricultural emissions to continue to rise.

Not all farming practices contribute to emissions equally, however. In fact, there is a growing awareness of farming techniques that remove carbon from the atmosphere, a process known as carbon sequestration, storing it in plant material and soils.

Locking carbon on the farm

During photosynthesis, plants absorb carbon dioxide. As they grow, the carbon is stored in their stems, leaves and roots. Once the plant dies or its leaves drop, soil microbes break down the material they leave behind. Some carbon is then re-released into the atmosphere, but a percentage can become stabilized and locked in the soil.

Scientists have found that certain farming practices can increase the amount of carbon sequestered in the soil. These include zero tillage, in which the soil isn’t disturbed by being cultivated or turned over; crop rotation, or growing different types of crops over several seasons on the same land; and cover cropping, where certain plants are grown primarily to benefit the soil rather than as a crop. As an added bonus, these practices that increase carbon sequestration often reduce soil erosion and help to retain important nutrients in the soil. The result is healthier soil and increased yields.

It was these potential “win-win” benefits that led a team of researchers headed by Stefan Frank, from the Austria-based International Institute for Applied Systems Analysis (IIASA), to model the potential benefits of implementing carbon-sequestering farming practices.

Using the Global Biosphere Management Model (GLOBIOM) developed by IIASA, the team found that storing carbon in soil could play an important role in reducing agricultural emissions. Combining a widespread change to carbon-sequestering farming practices with a carbon tax, GLOBIOM predicted a potential reduction in greenhouse gas emissions from agriculture of 11.4 gigatons of carbon dioxide equivalent per year. That’s 44 percent higher than the 7.9 gigatons achievable with only a carbon tax.

That difference of 3.5 gigatons of CO2e would be roughly the same as taking just over half the world’s cars, 583 million, off the roads.

There are caveats to the findings, though. Like any model, the accuracy of GLOBIOM’s output is ultimately determined by the assumptions underpinning it. As the study’s authors note, the reason predictive models have not previously included soil carbon sequestration is that it’s incredibly complex.

One of the key issues is the difficulty in measuring changes in soil carbon.

“Across a field … a change in 0.1 percent is a huge amount of carbon either being stored or released, but it’s really difficult to measure that change sensitively and accurately,” said Vanessa Bailey, a soil carbon expert from the U.S. Department of Energy’s Pacific Northwest National Laboratory, in Washington state, who was not involved in the study.

There is a great amount of variation in the soil carbon across a field, Bailey said. “If you take a sample in a depression there might be 1.1 percent carbon there just because we’ve had runoff there,” she said.

A peat bog being drained in Kalimantan. The lack of oxygen due to wet conditions in peat bogs prevents the peat from breaking down. When peat bogs dry out they start to decompose and large quantities of stored carbon are released to the atmosphere. Photo credit: Rhett Butler

The soil carbon cycle is also a two-way street. Under certain conditions, such as when peat is exposed and allowed to dry out, the soil becomes a major emitter of carbon.

Another issue is that soil can only capture so much carbon before it becomes saturated, a point known as soil carbon equilibrium.

This means that soil carbon sequestration may not be a long-term solution, but, according to Smith, could contribute “to a medium term solution while we fully decarbonise all sectors.”

Research has so far mainly focused on how to stabilize carbon in soil, but there is still a lot to learn about what destabilizes carbon in soil.

“We don’t have a good way of assessing how long this new carbon … persists in soils,” Bailey said. “That’s the other side of the coin we don’t understand.”

Because soil carbon sequestration is such a complex subject, scientists continue to debate exactly how much carbon could be stored in soils. Certainly no one believes soil carbon sequestration can mitigate all manmade emissions.

David Powlson, a leading soil scientist at Rothamsted Research in the U.K., who wasn’t involved in the study, was more cautious.

“I am concerned that many colleagues seem over optimistic about the amount of carbon that can be locked up this way,” he said.

In a recent study, Powlson evaluated the potential of the French government’s “4 per 1,000” initiative to increase global soil carbon stocks by 0.4 percent per year. Based on his research, Powlson said he believed agricultural practices that increase carbon sequestration “are limited by practical and economic factors.”

“[M]any ‘good practices’ are already being applied in many places, so scope for [expanding carbon-sequestering farming practices] is further limited,” he said.

Deforestation in Belize for cattle ranching. Land use change is one of the major sources of greenhouse gas emissions. Photo credit: Rhett Butler

The food security wrench

Another important area that the team was keen to investigate was the impact on food security of tax measures to cut greenhouse gas emissions.

Land-use change and agriculture, as major emitters, have long been the focus of much mitigation research. However, with the global population expected to reach 9.8 billion by 2050, scientists are faced with the difficult proposition of simultaneously reducing emissions even as the human population looks to continue growing.

A carbon tax would theoretically increase the cost of farming practices that produce high greenhouse gas emissions. This cost would then be passed on to consumers as higher food prices. As prices rise, producers and consumers will, in theory, alter their behavior and switch to products and practices with lower greenhouse gas emissions because they are cheaper.

At first glance this would seem like an ideal solution. But the GLOBIOM model highlights an important problem: If global warming is to be limited to 1.5 degrees Celsius by 2050, the aspirational target set in the Paris Agreement, then major changes will be required from the agriculture and land-use sector. Potential changes range from technical solutions such as the use of anaerobic digesters (which break down waste to produce fuel) to broader structural solutions, such as shifting production across regions.

According to Frank’s predictions, a carbon tax sufficient to stimulate this change in behavior would increase the cost of food so much that, theoretically, every person would consume on average 285 kilocalories less per day.

GLOBIOM predicts that over the next 20 years, economic development will reduce chronic undernourishment from its current level of 850 million to 200 million. However, with a carbon tax in place, the higher cost of food could reverse some of that progress, resulting in an additional 300 million people unable to meet their calorie requirements.

GLOBIOM’s predictions suggest that sequestering more carbon in soils could reduce the carbon tax required, cutting calorie loss by 65 percent while still hitting the 1.5-degree target.

It’s important to note, however, that when predicting scenarios without a carbon tax, GLOBIOM does not model the impact of climate change on food security. As Smith says, the food security consequences if we do not act on climate change will certainly be far worse than if we do.

“Massive losses of productivity for crops and livestock, mostly in countries already at risk of food insecurity,” he said, “so there is no option to just let it happen.”

Unequal risk, unequal rewards

As is so often the case with climate change, the impacts would be far from evenly distributed. For those in developed countries, where food costs a relatively small percentage of income, a hypothetical rise in price is not likely to greatly affect consumption. In the developing world, where food can cost a large proportion of people’s income, a rise in food costs could prove devastating.

There is also a major imbalance in the mitigation potential available to different countries depending on the type of agriculture and how much standing forest remains. In land-rich countries like Brazil, where a large proportion of emissions stem from converting rainforest to cropland or for grazing, targets can be met by limiting deforestation and forest degradation with little impact on food cost.

“Reducing emissions from land use change is a very cost-efficient and important strategy,” Frank said.

However, in densely populated countries like India, most agricultural emissions come from food production. The carbon tax required to stimulate agricultural change could have a major impact on the cost of food there.

With such stark regional differences, the authors emphasize the importance of a globally coordinated strategy. The GLOBIOM model predicts that any scenario without full global buy-in would result in a worse impact to food security than a globally coordinated approach.

For Frank, the show of global support for the Paris Agreement offers a glimmer of hope.

“Personally I hope that the momentum of the Paris Agreement will be maintained and even further strengthen[ed] so that we can achieve this tremendous challenge,” he said.

Cattle ranching in Colombia. Livestock production produces a large quantity of greenhouse gas emissions. The methane produced by cows is 30 times more heat trapping than carbon dioxide. Photo credit: Rhett Butler

No simple answers

Promoting farming practices that help store carbon in the soil could help offset the side effects of carbon tax policies, but more must be done to tackle climate change on a wider scale, Smith said.

“It’s not a climate solution in itself — but makes a valuable contribution toward addressing climate change,” he said.

“We really have to balance sequestration with mitigation with adaption to new conditions,” Bailey added. “That’s the three legs of a stool we all have to be aware of.”

For Powlson, changes in human consumption patterns and behavior are also key.

“It’s almost certainly necessary that we reduce food consumption and eat less meat,” he said.

Although the exact findings of any model can be open to debate, this study emphasizes the important relationship between soil carbon, food security and tackling climate change.

“There are no magic bullets,” Smith said. “It will be tough, but it has to be done.”